The molding of pre-expanded polymeric beads (e.g., expanded polystyrene--a.k.a. EPS) into foamed articles such as drinking cups, "lost-foam" molding patterns, Christmas decorations, etc., is a well known process wherein partially pre-expanded beads are blown into a mold, and therein subjected to steam to complete their expansion and fuse them together into a unitary mass. Expandable polystyrene is commercially available in the form of relatively small (e.g., 0.25 mm diameter, 40 lbs./ft..sup.3 density) white beads. The beads are formed of a suitable grade of polystyrene homopolymer for the intended molding purpose. Distributed throughout each polystyrene bead is an amount, usually about 5.5 to 6.5 percent by weight, of a suitable vaporizable expanding agent such as the hydrocarbon pentane. A portion of the pentane is probably dissolved in the polymer matrix of the bead, but a major portion of the pentane is distributed in microcavities throughout the polystyrene bead.
Before articles can be molded from the beads, the beads are subjected to a pre-expansion operation in which they are expanded and reduced in density by heating. Pre-expansion equipment is readily available commercially. In one pre-expansion process, a group of the beads is conveyed into a closed cavity where the beads are contacted with saturated steam at low superatmospheric pressure. The steam heat produces an expansion of the beads so that their diameter is increased, e.g., about fourfold, and some of the expanding agent, the pentane, is lost. The beads are discharged into a fluidized bed where room temperature air fluidizes, dries and cools the beads. At the conclusion of the pre-expansion step, the density of the beads is typically in the range of 1 to 1.6 lbs./ft..sup.3, and the content of the pentane at this stage is suitably about four to five percent by weight of the bead. The diameter of the bead is now about 1 mm. The expanded bead has a cellular structure and is close to the size at which it can be suitably molded into a finished article, e.g., a "lost foam" foundry pattern. In another version of the pre-expansion process, the beads are drawn into a space which is evacuated, where they are heated at about 200.degree. F. in the vacuum to accommodate the expansion of the beads. The expansion of the beads is arrested by the addition of water to the system. The water flashes in the vacuum, cooling the beads prior to discharge from the vacuum vessel. After this stage, the expanded beads are typically screened to remove any of the raw beads that fail to undergo the expansion process or any clumps of beads that are stuck together.
Molding of the beads into a finished article follows. The mold used to shape the foamed article has perforate walls defining a mold cavity and through which the steam enters the mold cavity. The mold is sandwiched between a pair of steam chests for applying the steam to the mold. The molds typically comprise separable mold segments/inserts inserted in and clamped, or otherwise affixed, to the steam chests. The use of mold inserts permits a single steam chest to be used with a variety of different molds for making a variety of different articles.
In the "flow-through" steaming technique, steam is introduced, through one of the mold segments on one side of the mold cavity, passes through the bed of beads in the mold cavity, and exits the mold cavity through the other mold segment on the opposite side of the mold cavity. An alternative steaming technique is known as "autoclaving" which involves pressurizing both steam chests at the same time so as to soak the beads in the steam for a sufficient duration to expand and fuse the beads together. Some practitioners use a combination of both the flow-through and the autoclaving techniques to insure rapid, uniform heating and bonding of the beads.
Following steaming, the molded article is cooled by spraying water onto the backside of the mold segments and/or by the application of vacuum to the steam chests until the expansion of the beads is arrested. In the case of EPS, steaming occurs at about 240.degree. F. Thereafter, the mold is cooled to about 140.degree. F. before bead expansion is arrested. The precise amount of time needed for steaming and cooling will vary with the size and complexity of the particular article being molded and the uninsulated mold mass. The mold is then opened, and the molded article ejected, e.g., by means of compressed air, mechanical ejection pins or the like. The mold is then closed and the cycle repeated.
In order to increase productivity of the apparatus, it is necessary to shorten the time required to complete the aforesaid operational cycle. One impediment to shortening the cycle time is the time required to heat-up and cool-down the mold segments, steam chest, and plumbing attachments thereto. In this regard, while the beads are being steamed, the surrounding metal forming the molding apparatus heats up and must then be cooled by as much as 100.degree. F. or more before ejection of the article from the mold can occur. Another impediment to shortening the cycle time is the time required to build-up steam pressure to flow through the beads in the mold during the steaming cycle as well as to build-up a vacuum to cool down the mold and molded article.
Heretofore, the cycle time as well as the energy requirements of the molding apparatus has been reduced by: (1) reducing the mass of the systems components, (2) making the molds and steam chests from aluminum which has a relatively low specific heat for quicker heating and cooling with relatively low energy consumption; and (3) lining the steam chest with an insulating material to reduce the heat transfer from the steam cavity to the metal (e.g., aluminum) forming the steam chest. While these techniques have improved the cycle time and reduced the required energy load, further cycle-time reductions are desirable.
Accordingly, it is an object of the present invention to further reduce the operational cycle time of an expanded polymer bead molding apparatus by reducing the amount of heat conducted between the steam chests and the mold segments attached thereto.
This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.